Technical Field
This Patent Disclosure relates generally to multistep resistor attenuators, such as for use as a front-end signal attenuator.
Related Art
A multistep programmable resistor attenuator is configured from resistors and transistor switches, with transistor switching controlled to provide programmable step attenuation. The switches control current flow, and when on (conducting) are characterized by an Ron resistance.
FIG. 1 illustrates a current mode programmable resistor step attenuator, configured as a front-end signal attenuator for a differential amplifier. Signal current flows either to differential ground through the differential switches, or to the virtual grounds at the differential non-inverting and inverting inputs VGP,VGM. For example, a 6 dB step attenuator can be configured with Ra=Rs/2, and Rb=Rs.
Transistor non-linearities produce signal distortion in the form of extraneous frequency content. For example, transistors can produce significant low-order HD2 and HD3 harmonic distortion, with HD3 distortion particularly problematic in differential configurations.
Switch distortion can be reduced by increasing switch size, but that approach increases area and parasitics on the virtual ground.